The Mattagami flows 443 kilometres (275 mi) from its source at Mattagami Lake in geographic Gouin Township[1] in the Unorganized North Part of Sudbury District, on the Canadian Shield southwest of Timmins,[2] to Portage Island in geographic Gardiner Township[3] in the Unorganized North Part of Cochrane District, in the Hudson Bay Lowlands.

[5] The Mattagami River flows through the city of Timmins as well as the town of Smooth Rock Falls[5] and its drainage basin encompasses 37,000 square kilometres (14,000 sq mi).

[2] It is close to tourism sites offering activities such as fishing, canoeing and nature-based relaxation.

The Mattagami's name comes from the Ojibwe and means either "the start of water" (maadaagami) or "turbulent water" (madaagami), but the local Ojibwe population claim "Mattagami" is a corrupted form of "confluence" (maadawaagami).

[6] Where the Groundhog and Kapuskasing Rivers flow into the Mattagami, Ontario Power Generation operates the Little Long Generating Station,[7] with a dam just over 5 km in length.